Combined Power and Heating Station

ABSTRACT

A combined heat and power plant has at least one primary heat source thermally connected to a heat distribution network for heat energy via one or more primary heat exchangers. At least one secondary heat source is thermally connected to one or more energy converters arranged to, when an amount of heat energy is supplied from the at least one secondary heat source, generate an amount of electrical energy for an internal electricity distribution network in the combined heat and power plant. A method is for operating a combined heat and power plant.

A combined heat and power plant (CHP plant) is described, in which aprimary heat source is in thermal contact with a heat distributionnetwork for heat energy via a primary heat exchanger. A method ofoperating a CHP plant is described as well.

Modern district heating stations use, to a great extent, biomass as theenergy source, the biomass being burnt and the energy released beingutilized to heat water to a suitable temperature. To avoid coming underregulatory requirements, for example, for steam boilers or the like,such district heating plants are, as a rule, operated at a temperaturenot higher than 120° C. and a working pressure of up to 2 bars. In spiteof their, in part, large energy production, plants cannot be operated asindependent units as they must have electrical energy from an externaldistribution network for the operation of pumps et cetera.

The invention has for its object to remedy or reduce at least one of thedrawbacks of the prior art or at least provide a useful alternative tothe prior art.

The object is achieved through features, which are specified in thedescription below and in the claims that follow.

The invention provides a combined heat and power (CHP) plant which, inaddition to producing heat at a relatively tow temperature (up to 120°C.), is self-sufficient in electrical energy and can possibly alsodeliver a surplus of electrical energy to an external electricitydistribution network. This is achieved by the CHP plant including aprimary heat source, in which the combustion of a fuel, typically bybiomass being burnt, provides for a primary heat-distribution fluid toachieve a prescribed temperature, typically up to 120° C., forcirculation in a heat-distributing network, also called a districtheat-distribution network, and in which, in addition to the primary heatsource, a secondary heat source is arranged, in which the combustion ofa fuel, typically the same type of fuel as in the primary heat source,provides for a secondary heat-distribution fluid to be heated to atemperature higher than that of the primary heat-distribution fluid, inorder thereby to provide a more high-grade heat energy, which is usedfor the operation of an energy converter, typically a heat engine whichdrives an electric generator, or a thermoelectric generator whichconverts heat energy into electrical energy. The electrical energy isused for the operation, of the primary heat source and possibly all orparts of the heat-distribution network. In addition, surplus energy maybe transmitted to an external distribution network for electrical energyvia an electrical Interfacing device, which may also be used when theCHP plant has a deficiency of self-produced electrical energy, forexample in a situation with a shutdown of the energy converter or thesecondary heat source.

It may also be a purpose of the invention to utilize residual heat fromthe energy converter, this residual heat being transmitted, to thegreatest extent possible, to the primary heat-distribution fluid that isused to transmit heat energy from the CHP plant to consumers connectedto an associated heat-distribution network. This may be achieved bymeans of one or more heat exchangers that transmit(s) residual heat fromthe energy converter to the primary heat-distribution fluid by all orparts of the primary heat-distribution fluid circulating through a heatexchanger in the energy converter.

The proportion between the primary and the secondary heat sources maytypically lie in the range between 2:1 and 20:1, wherein, typically, theaim is to provide approximately 5 kW of electrical energy from theenergy converter.

The secondary heat source will typically deliver heat in the range of150-300° C. distributed by means-of heated thermo-oil, pressurized wateror the like, which in turn is circulated to the energy converter. Inperiods of less need for heat energy, it is conceivable that only thesecondary heat source is kept operative, as surplus heat (residual heat)from the energy converter provides a sufficient energy supply to theheat-distribution network.

In some cases, the electrical energy supplied from the energy convertermay also be distributed on the external electricity distributionnetwork, before it is then directed back to the internal electricitydistribution network. This is generally done in cases in which ametering system records energy in versus energy out, and separate meterscan thereby be used to monitor the energy flow into and out of thesystem. In several existing European small hydro-electric powerstations, that is the way it works today. In other words, all the powerproduced is first fed out on the external network, as power consumed inthe internal network is always taken from the external network. In thatrespect, the electrical interfacing device may be restricted to formingonly an interface between the energy converter and the externalelectricity distribution network, as the internal electricitydistribution network will than always be directly connected to theexternal electricity distribution network. There are also mere ways ofconfiguring the electricity distribution flow in and at a CHP plant, andthe invention is not limited to one specific configuration.

The primary heat source may also include heat sources which are notbased on combustion, for example various waste-heat sources orgeothermal heat sources.

The heat-exchanger fluid circuit which is connected to the secondaryheat source may also be provided with a bypass heat exchanger, which cantransfer heat directly to the heat distribution network whenever needed,for example during a shutdown of the energy converter.

For heat conveyance in general, several types of thermofluids can beused, and the most obvious ones will be water, air or various industrialheat-transmission fluids such as various thermo-oils or silicone oils.

In a simple embodiment, the heat distribution network may consist of anair inlet, a fan, pipes and heat exchangers and also a hot-air outlet,wherein the hot air may be used for heating or, for example, drying ofdifferent materials, for example biomass. In such a ease, the heatdistribution network will be an open system, as the thermofluid, theair, that is, will be exchanged with the atmosphere all the time, asopposed to closed systems, such as most local/district heating plantswhich often use a closed circuit with water as the thermofluid.

In a first aspect, the invention relates more specifically to a combinedheat and power plant for a district heating plant or a local heatingplant in which at least a primary heat source in the form of a biomassfuel burning boiler is thermally connected to a heat distributionnetwork for distribution of heat energy at a temperature not higher than120° C. via one or more primary heat exchangers, characterized by atleast one secondary heat source fn the form of a biomass fuel burningboiler being thermally connected to one or mom energy converters in theform of one or more heat engines arranged to generate, when an amount ofheat energy at a temperature considerably higher than 120° C. issupplied from the at least one secondary heat source, an amount ofelectrical energy for an internal electricity distribution network inthe CHP plant.

The internal electricity distribution network and an externalelectricity distribution network may be electrically interconnected viaan electrical interfacing device arranged to transmit at least parts ofthe generated amount of electrical energy from said energy converter(s)to the external electricity distribution network.

The electrical interfacing device may be arranged for transmitting anamount of electrical energy at least corresponding to the amount ofelectrical energy generatable in said energy converter from the externalelectricity distribution network to the internal electricitydistribution network.

The heat distribution network may include at least one tertiary heatexchanger, which is thermally connected to one or more of said energyconverters and is arranged for the transmission of an amount of residualheat energy from said energy converter(s).

Said tertiary heat exchanger(s) may be arranged upstream of said primaryheat exchanger(s).

An air pre-heater may be thermally connected to one or more of saidenergy converters and may be arranged to receive a portion of an amountof residual heat energy from said energy converter(s).

The ratio between the nominal thermal power capacity of the primary heatsource and the nominal thermal power capacity of the secondary heatsource may be in the range of 2:1-20:1.

In a second aspect, the invention relates more specifically to a methodof operating a combined heat and power (CHP) plant for a districtheating plant or a local heating plant, characterized by the methodincluding the following steps:

a) providing an amount of heat energy from at least one primary heatsource in the form of a biomass fuel burning boiler at a temperature nothigher than 120° C. for one or more heat-energy consumers connected to aheat distribution network,

a1) in order to, by thermal contact between at the least one primaryheat source and the heat distribution network via one or more primaryheat exchangers, transmit heat energy from said primary heat source(s)to a thermofluid in the distribution network;

a2) by thermal contact between the at least one secondary heat source inthe form of a biomass fuel burning boiler and the heat distributionnetwork, supplying an amount of heat energy via at least one tertiaryheat exchanger in the form of residual heat energy from the conversion,by said energy converter(s), into electrical energy of the amount ofheat energy supplied from said secondary heat source;

b) by means of one or more energy converters in the form of one or moreheat engines, converting into electrical energy an amount of heat energywhich is supplied to said energy converter(s) from at least onesecondary heat source at a temperature considerably higher than 120° C.;

c) transmitting the electrical energy from said energy converter(s) toan internal electricity distribution network;

d) when there is a surplus of electrical energy, transmitting to anexternal electricity distribution network a portion of the generatedelectrical energy via an electrical interfacing device; and

e) when there is a deficiency of electrical energy, transmittingelectrical energy from the external electricity distribution network tothe internal electricity distribution network via the electricalinterfacing device.

The method may include the further step of:

f) supplying the amount of heat energy via said tertiary heatexchanger(s) upstream of said primary heat source(s).

The method may include the further step of:

g) supplying heat energy to an air supply for said primary heatsource(s) by means of an air pre-heater, the heat energy being, at leastin part, residual heat energy from said energy converter(s).

In what follows, an example of a preferred embodiment is described,which is visualized in accompanying drawings, in which:

FIG. 1 shows a principle drawing of a district heating plant accordingto the prior art;

FIG. 2 shows a principle drawing of a first embodiment of a combinedheat and power plant according to the invention;

FIG. 3 shows a principle drawing of a second embodiment of a combinedheat and power plant according to the invention; and

FIG. 4 shows a principle drawing of a third embodiment of a combinedheat and power plant according to the invention.

Reference is made in particular to the FIGS. 2, 3 and 4 as regards thedescription of an exemplary embodiment of the present invention. Theprior art, as it appears from FIG. 1, exhibits some principal featuresshared by the invention, and like elements are indicated by the samereference numerals.

The reference numeral 1 indicates a combined heat and power (CHP) plantaccording to the invention. A primary heat source 11 is connected to aheat distribution network 15 arranged to deliver heat energy Q at a heatconsumer 16. The primary heat source 11 may be a hosier normallydimensioned per se, having a nominal output of 0.1-1 MW. The primaryheat source 11 may be heated through the burning of a supply fuel 18suitable therefor, for example biomass, under a supply of air 181, anamount of heat energy Q_(L1) being made available to a primary heatexchanger 111 which forms part of the circulation circuit formed by theheat distribution network 15. Via the primary heat exchanger 111, theprimary heat source 11 heats a first heat distribution fluid suitabletherefor, for example water or a thermo-oil, which circulates in theheat distribution network 15 with an outlet temperature limited toapproximately 120° C. at a pressure not exceeding approximately 2 bars.

A secondary heat source 12 may be a smaller boiler with a nominal outputof approximately 50 kW. The secondary heat source 12 may be heatedthrough the burning of a fuel 18′ suitable therefor, for example thesame type of fuel as that used by the primary heat source 11, under asupply of air 181, an amount of heat energy Q_(L2) being made availableto a first secondary heat exchanger 121. The secondary heat source 12heats a second heat distribution fluid suitable therefor, for examplewater under overpressure or a thermo-oil, circulating in aheat-exchanger fluid circuit 122, to an outlet temperature considerablyhigher than 120° C., typically 150-300° C. An amount of heat energyQ_(H) is transmitted to a second secondary heat-exchanger 131 in anenergy converter 13, typically formed as a heat engine or athermoelectric generator, which, by means of the heat energy Q_(H)supplied, generates electrical energy P_(EL), typically at a nominalpower output of approximately 5 kW. The electrical energy P_(EL) whichis delivered to an infernal electricity distribution network 19, is usedfor the operation of electrical components (not shown) connected to theprimary heat source 11 and possible other electric consumers in the CHPplant 1.

An electrical interfacing device 14, for example an inverter, isconnected to the internal electricity distribution network 19 in the CHPplant 1 and an external electricity distribution network 17 in such away that a surplus of electrical energy P_(EL) from the energy converter13 can be supplied to the electricity distribution network 17, and adeficiency in energy P_(EL) from the energy converter 13 can be coveredby a supply from the electricity distribution network 17, for example ina situation in which a shutdown of the secondary heat source 12 or theenergy converter 13 will require a supply from externalelectrical-energy sources.

The heat distribution network 15 forms a closed circuit for thecirculation of the first heat distribution fluid and the transmission ofan amount of heat energy Q to one or more nest-energy consumers 16,shown schematically here as one heat-energy consumer 16. In theembodiment shown in the FIGS. 3 and 4, the heat distribution network 15is additionally connected to a tertiary heat exchanger 151 arranged inthe energy converter 13 and arranged to transmit residual heat energyQ_(L) from the energy converter 13 and thereby preheat the return flowof the cooled, first heat distribution fluid circulating in the heatdistribution network 15. The residual heat energy Q_(L) isadvantageously supplied upstream of the primary heat exchanger 111 tothe primary heat source 11 to achieve a lowest possible heat-sinktemperature for the energy converter 13 and thereby high efficiency.

It may be an advantage to preheat the air supply 181 to increase theefficiency of the primary heat source 11 in particular. For thispurpose, an air pre-heater 182 may be used, which, in the embodimentshown according to FIG. 4, gets heat supplied from the heat distributionnetwork 15 which, downstream of the tertiary heat exchanger 151, is laidin a loop passing through the air pre-heater 182. In an embodiment notshown, the air pre-heater 182 may be connected to a separate heatdistribution circuit (not shown) which is in thermal contact with theenergy converter 13, for example via the tertiary heat exchanger 151 ora further heat exchanger (not shown) arranged in connection with theenergy converter 13 for the transmission of a portion of the residualheat energy Q_(L).

When there is little need for heat energy, for example in summer, thesupply of residual heat energy Q_(L) by the energy converter 13,according to the embodiments shown in the FIGS. 3 and 4, may besufficient to cover the demand for heat energy Q. In such a situation,the operation of the primary heat source 11 may be stopped. When theoperation of the primary heat source 11 is stopped, the need forelectrical energy P_(EL) will decrease, and the residual heat energyQ_(L) available may be increased.

The proportion between the primary and secondary heat sources 11, 12 istypically 2:1-20:1, that is to say the primary heat source 11 may beprovided with a burner of an ordinary size for this type of plant, thatis to say typically between 100 and 1000 kW, whereas the secondary heatsource is provided with a relatively small burner, typically ofapproximately 50 kW.

The CHP plant 1 according to the invention may be used for a greatnumber of purposes in which heat energy generated by the combustion of afuel is to be distributed. Typical areas of application are plants forburning waste, biomass, regular district heating stents, wood driers(chips, shavings and sawn-timber driers) and so on.

1. A combined heat and power plant (1) in which at least one primaryheat source (11) is thermally connected to a heat distribution network(15) for heat energy (Q) via one or more primary heat exchangers (111),characterized in that at least one secondary heat source (12) isthermally connected to one or more energy converters (13) arranged to,when an amount of heat energy (Q_(H)) is supplied from the at least onesecondary heat source (12), generate an amount of electrical energy(P_(EL)) for an internal electricity distribution network (19) in thecombined heat and power plant (1).
 2. The combined heat and power plant(1) according to claim 1, wherein the internal electricity distributionnetwork (19) and an external electricity distribution network (17) areelectrically interconnected via an electrical interfacing device (14)arranged for the transmission of at least parts of the generated amountof electrical energy (P_(EL)) from said energy converter(s) (13) to theexternal electricity distribution network (17).
 3. The combined heat andpower plant (1) according to claim 2, wherein the electrical interfacingdevice (14) is arranged for the transmission of an amount of electricalenergy at least corresponding to the amount of electrical energy(P_(EL)) generatable in said energy converter(s) (13) from the externalelectricity distribution network (17) to the internal electricitydistribution network (19).
 4. The combined heat and power plant (1)according to claim 1, wherein the heat distribution network (15)includes at least one tertiary heat exchanger (151) which is thermallyconnected to (one of) said energy converter(s) (13) and is arranged forthe transmission of an amount of residual heat energy (Q_(L)) from saidenergy converter(s) (13).
 5. The combined heat and power plant (1)according to claim 4, wherein said tertiary heat exchanger(s) (151)is/are arranged upstream of said primary heat exchanger(s) (111).
 6. Thecombined heat and power plant (1) according to claim 1, wherein an airpre-heater (182) is thermally connected to one or more of said energyconverters (13) and is arranged to receive a portion of an amount ofresidual heat energy (Q_(L)) from said energy converter(s) (13).
 7. Thecombined heat and power plant (1) according to claim 1, wherein theratio between the nominal thermal power capacity (Q_(L1)) of the primaryheat source (11) and the nominal thermal power capacity (Q_(L2)) of thesecondary heat source (12) is in the range of 2:1-20:1.
 8. A method ofoperating a combined heat and power plant (1), characterized in that themethod includes the following steps: a) providing an amount of heatenergy (Q) for one or more heat-energy consumers (16) connected to aheat distribution network (15), a1) in order to, by thermal contactbetween at least one primary heat source (11) and the heat distributionnetwork (15) and via one or more primary heat exchangers (111),transmitting heat energy from said primary heat source(s) (11) to athermofluid (152) in the distribution network (15); b) by means of oneor more energy converters (13), converting into electrical energy(P_(EL)) an amount of heat energy (Q_(H)) which is supplied to saidenergy converter(s) (13) from at least one secondary heat source (12);c) transmitting the electrical energy (P_(EL)) from said energyconverter(s) (13) to an internal electricity distribution network (19);d) when there is a surplus of electrical energy, transmitting to anexternal electricity distribution network (17) a portion of thegenerated electrical energy (P_(EL)) via an electrical interfacingdevice (14); and e) when there is a deficiency in electrical energy,transmitting electrical energy from the external electricitydistribution network (17) to the internal electricity distributionnetwork (19) via the electrical interfacing device (14).
 9. The methodin accordance with claim 8, wherein the method includes the further stepof: a2) by thermal contact between the at least one secondary heatsource (12) and the heat distribution network (15), supplying an amountof heat energy via at least one tertiary heat exchanger (151) in theform of residual heat energy (Q_(L)) from the conversion, by said energyconverter(s) (13), into electrical energy (P_(EL)) of the amount of heatenergy (Q_(H)) supplied from said secondary heat source (12).
 10. Themethod in accordance with claim 9, wherein the method includes thefurther step of: f) supplying the amount of heat energy via saidtertiary heat exchanger(s) (151) upstream of said primary heatsource(s).
 11. The method in accordance with claim 8, wherein the methodincludes the further step of: g) supplying heat energy to an air supply(181) for said primary heat source(s) (11) by means of an air pre-heater(182), the heat energy being, at least in part, residual heat energy(Q_(L)) from said energy converter(s) (13).